The reason the available funds on your Credit Builder Loan fluctuate is because of the interest. Interest is calculated by taking the principal balance owing and multiplying it by the daily interest rate (the interest rate of the loan divided by 365 days in a year). The interest then builds up and reduces your available funds until your next loan payment pays off the accrued interest and any additional services you have, such as Black Tier or Loan Protection Plan.
Your payment is applied to the interest owing first, then the remaining amount of the payment is applied to the principal as part of your available funds. Month over month, your available funds will increase with each payment, even if during the course of the month they appear to decrease due to interest being added.